1. Field of the Invention
The present invention relates to coated optical fibers and, more specifically, to an improved manner of color coding the coated optical fibers wherein the manner of color coding also produces faster cure speeds in the optical fiber coating.
2. Related Art
Traditionally, the pigments used to color code optical fibers block light. Because the traditional pigments block light, a low amount of ultraviolet (UV) radiation is allowed to pass through the coating containing the traditional pigments. The amount of UV radiation allowed to pass through the pigment containing layer directly affects cure time of the pigment containing layer as well as any other curable layers underneath thereof. Therefore, the use of traditional pigments is an impediment to increased curing time of optical fiber coatings.
U.S. Pat. No. 5,259,060 to Edward et al. discloses a coated optical fiber which includes an ink layer, and a layer containing an opaque white pigmented layer. The opaque white layer includes sufficient white pigment to render it light-reflective and substantially opaque to visible light. However, the pigments used to produce the opaque white layer are traditional pigments which block UV radiation and, therefore, are an impediment to faster curing speeds.
An object of the present invention is to overcome the disadvantages of the prior art. More specifically, an object of the present invention is to increase cure speed of optical fiber coatings while maintaining easily identifiable color coding. A further object is to increase cure speed of optical fiber coatings while enhancing the brightness, or visibility, of the colors in an optical fiber coating.
In the present invention, a perlescent pigment is used in the polymer coating materials of an optical fiber to achieve easy color identification of the optical fiber, as well as to achieve an increased amount of UV transmissibility which thereby produces faster curing speeds. The present invention is of particular importance when the optical fiber contains a hermetic coating. The hermetic coating acts to protect the fiber from the potentially harmful effects of water vapor or gaseous contaminants affecting optical transmission characteristics or fiber strength. The hermetic coating is typically a metallic or carbon-based coating which tends to absorb light and present a dark background. Therefore, when present, the hermetic coating makes effective color coding even more difficult.
An important fact is that, due to the size and geometry of the fibers, much of the apparent brightness or color intensity in polymer fiber coatings is derived from light which is transmitted or reflected through the polymer matrix wherein the colorant resides. This includes a substantial light component traversing the coating which would normally be reflected from the surface of the fiber itself, but for the presence of the hermetic coating thereon.
Increasing the proportion of light transmitted through the coating is achieved in accordance with the invention by minimizing or avoiding the inclusion of additional opacifying agents in the pigmented layer, i.e., substituting a perlescent pigment material for the traditional pigments used for coloring optical fiber coatings. A perlescent pigment is used in the same coating as the colored pigments, or in a layer under the pigmented layer, to thereby increase the amount of light impinging on the colored pigment. By increasing the amount of light impinging on the colored pigments, the brightness or visibility of the pigment present in the colored layers is intensified.
Another important fact is that traditional pigments block UV radiation which is necessary to cure the polymers in and under the layer containing the pigments. Therefore, replacing a portion of the traditional pigments with perlescent pigments, which block significantly less radiation at wavelengths below about 400 nm, reduces the amount of UV radiation which is blocked. Because a decreased amount of UV radiation is blocked, more UV radiation is transmitted to cure the underlying polymer which, in turn, increases the curing speed. The perlescent pigment may include, for example, mica particles, mica particles coated with a pigment, fish scales, or any other type of particle that gives a metallic-looking finish to the polymer in which it is disposed.
In one embodiment, the perlescent pigment is made of mica particles. Mica particles are more transparent to UV radiation below about 400 nm, whereas wavelengths of about 250 to about 400 nm are used to cure the polymer coatings. Thus, the mica particles transmit more of the UV radiation wavelengths used for curing the polymer coating. Further, mica particles reflect light above about 420 nm, including light in the visible range and, therefore, enhance the brightness or visibility of color for fiber identification. Thus, the mica particles allow the underlying polymer coatings to be cured at a fast rate while at the same time producing an enhanced, easily identifiable, color in the optical fiber.
The average width of the mica particles may range from 1 xcexcm to the order of thickness of the coating in which they are contained. For example, when the thickness of the coating is on the order of 15 xcexcm, particles having an average width larger than 15 xcexcm would not work well. Preferably, the average width of the mica particles is from 1 xcexcm to 10 xcexcm which produces good results without producing additional stress on the fiber core, and without clogging the dies with which the optical fiber is produced. Most preferably, the average width of the mica particles is 8 xcexcm.
The mica particles may be coated with a form of titanium oxide (TiO2) to produce a xe2x80x9cwhitexe2x80x9d color. Two forms of TiO2 which work well, in particular, are rutile and anatase. A xe2x80x9cwhitexe2x80x9d perlescent pigment is useful in reflecting light to enhance the brightness and visibility of other colored pigments. Further, when used alone, the perlescent pigment produces an easily identifiable xe2x80x9cwhitexe2x80x9d colored fiber.
The composition of the polymers used in the optical fiber coatings in accordance with the present invention is not critical. Any of the well-known curable liquid polymers known to have utility for the application of optical fiber coatings may be employed. Examples of some well-known polymer types which have previously been used for protective and/or colored optical fiber coatings include urea-acrylate, urethane-acrylate, epoxy-acrylate, polyether acrylate, polyester acrylate, urethane epoxides, silicone acrylates, and silicone polymers. Either thermally curable or radiation curable forms of these coatings may be used. The polymer used to coat the optical fibers may be a UV curable polymer, an electron beam curable polymer, or any other suitable polymer. However, the perlescent pigments of the present invention are particularly useful with UV curable polymers. Therefore, the specification describes the present invention in connection with optical fiber coatings made of UV curable polymers, with the understanding that the polymer may be of any suitable type.